This invention is in the field of abrasive articles. More specifically, this invention relates to abrasive articles in which a powder of fusible particles is dry coated, liquefied, and then cured to form at least a portion of the bond system of the abrasive article.
Coated abrasive articles generally comprise a backing to which a multiplicity of abrasive particles are bonded by a suitable bond system. A common type of bond system includes a make coat, a size coat, and optionally a supersize coat. The make coat includes a tough, resilient polymer binder that adheres the abrasive particles to the backing. The size coat, also including a tough resilient polymer binder that may be the same or different from the make coat binder, is applied over the make coat to reinforce the particles. The supersize coat, including one or more antiloading ingredients or perhaps grinding aids, may then be applied over the size coat if desired.
In a conventional manufacturing process, the ingredients that are used to form the make coat are dispersed or dissolved, as the case may be, in a sufficient amount of a solvent, which may be aqueous or nonaqueous, to provide the make coat formulation with a coatable viscosity. The fluid formulation is then coated onto the backing, after which the abrasive particles are applied to the make coat formulation. The make coat formulation is then dried to remove the solvent and at least partially cured. The ingredients that are used to form the size coat are also dispersed in a solvent, and the resultant fluid formulation is then applied over the make coat and abrasive particles, dried and cured. A similar technique is then used to apply the supersize coat over the size coat.
The conventional manufacturing process has some drawbacks, however, because all of the coating formulations are solvent-based. Typical make and size coat formulations may include 10 to 50 weight percent of solvent. Supersize coating formulations, in particular, require even more solvent in order to form useful coatings having the desired coating weight and viscosity. Solvents, however, can be expensive to purchase and/or to handle properly. Solvents also must be removed from the coatings, involving substantial drying costs in terms of capital equipment, energy costs, and cycle time. There are also further costs and environmental concerns associated with solvent recovery or disposal. Solvent-based coating formulations also typically require coating methods involving contact with underlying layers at the time of coating. Such contact can disrupt the orientation of the coated abrasive particles, adversely affecting abrading performance.
Not surprisingly, solventless manufacturing techniques have been investigated. One promising approach involves powder coating techniques in which a coating is formed by dry coating a powder of extremely fine, curable binder particles onto a suitable backing, melting the coated powder so that the particles fuse together to form a uniform melt layer, and then curing the melt layer to form a solid, thermoset, binder matrix. For example, PCT patent publication WO 97/25185 describes forming a binder for abrasive particles from dry powders. The dry powders comprise thermally curable phenolic resins that are dry coated onto a suitable backing. After coating, the particles are melted. Abrasive particles are then applied to the melted formulation. The melted formulation is then thermally cured to form a solid, make coat binder matrix. A size coat may be applied in the same way. Significantly, the make and size coats are formed without any solvent, and the size coat powder may be deposited without contacting, and hence disrupting, the underlying abrasive particles.
Notwithstanding the advantages offered by powder coating techniques described in PCT patent publication WO 97/25185, the powders described in this document incorporate resins that are thermally cured. The use of such resins poses substantial challenges during manufacture. Thermally cured resins generally tend to be highly viscous at reasonable processing temperatures, and thus are difficult to get to flow well. This makes it somewhat challenging to cause the binder particles to melt and fuse together in a uniform manner. The thermally curable resins also typically require relatively high temperatures to achieve curing. This limits the kinds of materials that can be incorporated into an abrasive article. In particular, many kinds of otherwise desirable backing materials could be damaged or degraded upon exposure to the temperatures required for curing. It is also difficult to control the start and rate of thermal curing. Generally, thermal curing begins as soon as heat is applied to melt the powder particles. As a consequence, the cure reaction may proceed too far before the powder particles are adequately fused. Further, the resultant bond between the cured binder and the adhesive particles may end up being weaker than is desired.
Accordingly, there is still a need for a solventless manufacturing technique for making abrasive articles that avoids disrupting abrasive particle orientation as the various component layers of the abrasive bond system are formed.
The present invention involves the use of powder coating methods to form coated abrasives. In one embodiment, the powder is in the form of a multiplicity of binder precursor particles comprising a radiation curable component. In other embodiments, the powder comprises at least one metal salt of a fatty acid and optionally an organic component that may be a thermoplastic macromolecule, a radiation curable component, and/or a thermally curable macromolecule. In either embodiment, the powder exists as a solid under the desired dry coating conditions, but is easily melted at relatively low temperatures and then solidified also at reasonably low processing temperatures. The principles of the present invention can be applied to form make coats, size coats, and/or supersize coats, as desired.
The present invention offers several advantages. Firstly, because melting and curing occur at relatively low temperatures, abrasive articles prepared in accordance with the present invention can be used with a wider range of other components, for example, backing materials, that otherwise would be damaged at higher temperatures. The ability to use lower processing temperatures also means that the present invention has lower energy demands, making the invention more efficient and economical in terms of energy costs. Additionally, the powder coatings can be applied at 100% solids with no solvent whatsoever. Therefore, emission controls, solvent handling procedures, solvent drying, solvent recovery, solvent disposal, drying ovens, energy costs associated with solvents, and the significant costs thereof, are entirely avoided. Powder coating is a noncontact coating method. Unlike many solvent coating techniques, for example, roll coating or the like, powder coating methods are noncontact and, therefore, avoid the kind of coating contact that might otherwise disrupt coated abrasive particles. This advantage is most noticeable when applying size and supersize coats over underlying make coat and abrasive particles. Powder coating methods are versatile and can be applied to a broad range of materials.
The use of dry powder particles comprising a radiation curable component and/or a metal salt of a fatty acid is particularly advantageous in that excellent control is provided over the curing process. Specifically, one can precisely control not only when cure begins, but the rate of cure as well. Thus, the premature crosslinking problems associated with conventional thermosetting powders is avoided. The result is that a binder derived from binder particles and/or powders of the present invention tends to bond more strongly to abrasive particles and is more consistently fully fused prior to curing, making manufacture much easier. As another advantage, the binder particles of the present invention comprising a radiation curable component can be formed using low molecular weight, radiation curable materials that have relatively low viscosity when melted, providing much better flow and fusing characteristics than thermally curable, resinous counterparts.
In one aspect, the present invention relates to an abrasive article comprising a plurality of abrasive particles incorporated into a bond system, wherein at least a portion of the bond system comprises a cured binder matrix derived from ingredients comprising a plurality of solid, binder precursor particles, said binder precursor particles comprising a radiation curable component that is fluidly flowable at a temperature in the range from about 35xc2x0 C. to about 180xc2x0 C.
In another aspect, the present invention relates to a method of forming an abrasive article, comprising the steps of (a) incorporating a plurality of abrasive particles into a bond system; and (b) deriving at least a portion of the bond system from a plurality of solid, binder precursor particles, said binder precursor particles comprising a radiation curable component that is fluidly flowable at a temperature in the range from about 35xc2x0 C. to about 180xc2x0 C.
In still yet another aspect, the present invention provides a powder, comprising a radiation curable component that is a solid at temperatures below about 35xc2x0 C. and is fluidly flowable at a temperature in the range from about 35xc2x0 C. to about 180xc2x0 C.
The present invention also provides a fusible powder, comprising 100 parts by weight of a metal salt of a fatty acid and 0 to 35 parts by weight of a fusible organic component.
The present invention also relates to a method of forming a supersize coating on an underlying abrasive layer of an abrasive article. A fusible powder is dry coated onto the abrasive layer, wherein the fusible powder comprises at least one metal salt of a fatty acid. The fusible powder is liquefied to form a supersize melt layer. The supersize melt layer is solidified, whereby the supersize coating is formed.
As used herein, the term xe2x80x9ccured binder matrixxe2x80x9d refers to a matrix comprising a crosslinked, polymer network in which chemical linkages exist between polymer chains. A preferred cured binder matrix is generally insoluble in solvents in which the corresponding, crosslinkable binder precursor(s) is readily soluble. The term xe2x80x9cbinder precursorxe2x80x9d refers to monomeric, oligomeric, and/or polymeric materials having pendant functionality allowing the precursors to be crosslinked to form the corresponding cured binder matrix.
If desired, the cured binder matrix of the present invention may be in the form of an interpenetrating polymer network (IPN) in which the binder matrix includes separately crosslinked, but entangled networks of polymer chains. As another option, the cured binder matrix may be in the form of a semi-IPN comprising uncrosslinked components, for example, thermoplastic oligomers or polymers that generally do not participate in crosslinking reactions, but nonetheless are entangled in the network of crosslinked polymer chains.
As used herein, the term xe2x80x9cmacromoleculexe2x80x9d shall refer to an oligomer, a polymer, and combinations thereof.